1. Field of the Invention
The invention relates to an anode of an alloy of titanium containing elements from Groups 8 and 7B of the Periodic System, especially for the electrowinning of metals or metal compounds.
2. Description of the Prior Art
Electrodes are used in the electrochemical production of chlorine, for the electrowinning of metals from hydrochloric or sulphuric acid electrolytes, in electrolysis of water and other electrochemical processes. These electrodes consist essentially of a passivatable metal whose surfaces are coated with electrochemically active substances for improvement of activity, for example with platinum metals or platinum metal oxides. The activating layers are gradually worn away in the use of the electrodes or their catalytic effect lessens with the result that the electrodes must be reactivated after certain periods of time, that is the electrodes must be provided with a new coating. The operating life of the activation layer is especially short if solid electrolysis products such as manganese dioxide in electrolysis for pyrolusite production are deposited on the electrodes since parts of the activation layer are torn off. This tearing off of activation layer occurs chiefly during the periodic removal of the product. In principle, this loss of activation materials can be reduced by a firmer anchoring or binding of the activating substances to the electrode base. A certain degree of protection is achieved by porous construction of the electrodes and depositing of the activating substances in the pores.
It is also known to alloy the activating substances with the titanium basic substance of the electrode. The electrode basic substance is for this purpose coated with a platinum metal, and intermetallic compounds of the general formulae MeTi or MeTi.sub.3 which reach a few micrometers depth in the electrode are produced by heating of the coated electrode under reduced pressure, (German Offenlegungsschrift No. 26 45 414). Such diffusion layers are able to cope with stronger mechanical loads, although these electrodes are comparatively expensive on account of the high price of platinum metal. It is also known, to concentrate manganese on the surface of titanium anodes provided for the electrolytic production of manganese dioxide, the manganese being applied by diffusion, plasma spraying or sintering of powder mixtures of titanium-manganese (German Offenlegungsschrift No. 33 46 093). The manganese content on the surface of the titanium, preferably amounting to 20 to 60%, should drop within a layer thickness of 0.1 to 0.3 mm to 0%. In contrast to anodes which consist of titanium manganese alloy of the indicated composition over their entire thickness, these surface coated anodes are readily deformable and may be worked for example by rollers. Disadvantageously, the limited corrosion resistance of the anodes does not always suffice for the technical requirements to which the anodes are subjected. It is known to use alloys of titanium or another passivatable metal with chromium, manganese, rhenium, iron, cobalt, nickel, zinc, copper, silver, gold, cadmium, tin, lead, silicon, germanium, lanthanum or a platinum metal as the electrode (British Patent Specification No. 1 463 553 and equivalent U.S. Pat. No. 4,288,302). The electrodes are for example produced by shaping and sintering of the metal powder mixed in a given ratio of quantities and provided with a surface oxide layer which is formed as a result of a special oxidation treatment or in the intended use of the electrodes, for example in its use as an anode in an aqueous electrolyte. The electrodes are resistant to chemical attack and mechanical stressing and have a satisfactory electrochemical activity. The proportion of the alloy elements with these types of electrode amounts to 0.1 to 50%.
In another proposal, the electrode provided for the electrochemical recovery of metals consists of a titanium alloy which contains 51 to 55% nickel (U.S. Pat. No. 4,363,706). Above this region, the rate of corrosion of the anodes increases strongly, and below the region, the anode does not effectively cope with mechanical loading on account of its brittleness. The use of a high nickel content is however not always feasible in the electrodes, for example, the electrolysis product can be rendered impermissibly impure by nickel abrasion. The maximum permissible nickel content in electrolytically produced pyrolusites should not, for example, exceed a few ppm.